A boundary microphone (surface-mount sound pickup microphone) is also called a surface mount microphone because it is placed on a desk or floor for the use in, e.g., a TV studio and a conference. As described in Patent Document 1 (Japanese Utility Model Registration No. 2515812), a flat microphone case is used for a boundary microphone. An example will be discussed below with reference to the sectional view of FIG. 4 and the exploded sectional view of FIG. 5.
According to this example, a microphone case 1 used for a boundary microphone is basically constituted of two components of a metallic flat base 10 which is opened on its top surface and a metallic microphone cover 20 which has a number of openings (sound wave introduction holes) and is mounted on the base 10 so as to cover the top surface of the base 10.
In ordinary cases, the base 10 is formed by casting such as zinc die casting. A punching plate (perforated plate) is used as the microphone cover 20. A wire mesh body may be used instead of the punching plate. The microphone cover 20 is secured to the base 10 at one point by a screw. The microphone cover 20 is not secured at multiple points in consideration of its appearance.
To be specific, a screw insertion hole 21 is bored almost at the center of the microphone cover 20, a boss 11 having a female screw is raised on the insertion hole 21, so that the microphone cover 20 is fixed on the base 10.
The base 10 and the microphone cover 20 form a shield space in the microphone case 1. A circuit board 30 having a condenser microphone unit 31 mounted thereon is housed in the shield space. Moreover, an impedance converter, a tone control circuit, an output circuit, and the like (not shown) are mounted on the circuit board 30. A microphone code 32 is connected to the circuit board 30 and drawn from the base 10 through a code bushing 33.
Incidentally, the punching plate used as the microphone cover 20 is composed of, e.g., an iron plate on which a number of holes are made. The punching plate is cut into a predetermined shape and then pressed into a desired shape. Since the punching plate is a perforated plate, an edge face (cut face) 20a making contact with the base 10 becomes uneven. The casting surface of the base 10 also becomes uneven in the case of die casting. Thus, an electrical connection between the base 10 and the microphone cover 20 is point contact at multiple points.
Considering the influence of electromagnetic waves, the interference of electromagnetic waves in ordinary VHF and UHF band used in broadcast stations can be sufficiently handled by the shield of the base 10 and the microphone cover 20. However, in the field of microphones, the influence of strong electromagnetic waves of cellular phones has become a focus of attention as cellular phones have rapidly become widespread in recent years.
To be specific, when cellular phones are used, extremely strong electromagnetic waves are generated. (for example, in a range of about several cm to several tens cm, an electric field is several tens of thousands times as strong as an electric field generated by commercial radio waves).
In the shield of the base 10 and the microphone cover 20, an electrical contact is a point contact, so that the microphone cover 20 may act as an antenna and cause noise in the presence of the strong electromagnetic waves.